The average employer-provided health insurance premium notched up in the low single digits last year, continuing a relatively modest growth in the past five years that experts are still struggling to explain.

The 3 percent to 4 percent growth reported in 2012 was less than half of last year’s increase and is more in line with the figures reported in other years since the recession began in 2007, according to the Henry J. Kaiser Family Foundation’s much-anticipated annual Employer Health Benefits Survey, which was released Tuesday.

The cost of an average individual plan rose 3 percent to $5,615 annually, and the average family plan rose 4 percent to $15,745.

Double-digit premium increases were commonplace until 2004 — the last time one was recorded — and no one knows exactly what has caused the drop, KFF President and CEO Drew Altman wrote in an essay accompanying the report. The growth in high-deductible plans and the strain of the recession on household finances likely are contributing to the trend, he wrote.

“With the economy only slowly recovering and wage stagnation depressing utilization, there is no reason to expect a return to double-digit increases in health insurance premiums anytime soon, if at all,” Altman wrote.

The Health and Human Services Department was quick to suggest one possible factor. The agency released a report this morning that projected the Affordable Care Act’s rate review efforts have saved consumers about $2.1 billion.

The KFF report also found that the number of young adults remaining on their parents’ health insurance plan — one of the most popular provisions of the ACA — grew from 2.1 million in 2011 to 2.9 million in 2012.

Meanwhile, the number of grandfathered plans that are exempt from some regulations in the ACA fell to 48 percent of covered workers, down from 56 percent last year.